This invention applies to the field of micro-lithography and the application of micro-lithography as a tool in development and production of micro-electronic devices and integrated circuits.
Lithography, the printing of patterns on surfaces, is both a driving force and a bottleneck in semiconductor manufacturing and in microfabrication. Classical lithography is based on the concept of fidelity in reproducing the mask pattern onto the wafer. Shrinking critical dimension (CD) requirements in semiconductor manufacturing demand development of lithographic techniques producing desired patterns not necessarily being a mask pattern replica.
Next Generation Lithography (NGL) implies and is here defined as the departure from the classical concept of fidelity in the replication of masks. NGL replaces the narrower term-post optical lithography.
Among NGLs competing for sub-100 nm patterning, proximity X-ray lithography (PXL) is the most advanced and mature. PXL was first introduced as a post optical lithography and unavoidably inherited the classical tendency of replication fidelity. Restricted by this concept, PXL was thought to be limited to 70-75 nm imaging and printing capability using the smallest working mask/wafer gaps of 7-10 um for practical manufacturing.
This invention makes use of the modern control over development of resists.
It is an object of the invention to perform ultra high resolution proximity lithography by demagnifying mask features on printed wafers or other surfaces.
It is the second object of the invention to perform ultra high resolution proximity lithography with mask/wafer gaps larger than are used in the prior art for printed features of a given size.
It is the third object of the invention to reduce exposure times by developing at lower overall dose levels than in the prior art.
It is the fourth object of the invention to print dense, nested or half pitch features by using multiple exposures with stepping. Typically, the stepping will be blind.
It is the fifth object of the invention to achieve ultra high resolution with feature sizes down to 25 nm, and beyond, by applying the concept of NGL.
It is the sixth object of the invention to improve critical dimension (CD) control by the use of ultra high resolution proximity lithography, including demagnifying mask features on the printed wafers or other surfaces. The improved CD control arises firstly from the comparatively large gradient of the exposure dose at an optimised development level (e.g. level B in FIG. 3) with the addition of the following objectives.
It is the seventh object of the invention to reduce edge roughness in printed features by the use of ultra high resolution in proximity lithography. The edge roughness is reduced because, in Fresnel diffraction, the image results from interference due to rays from various parts of the clear mask feature in two dimensions resulting in smoothing at rough edges.
It is the eighth object of the invention to reduce defects in printed features due to other larger mask writing defects, including overlying dust particles, by the use of ultra high resolution proximity lithography. The mechanism is similar to the above.
It is the ninth object of the invention to reduce CD errors in printed features by the use of ultra high resolution proximity lithography employing highly sensitive, low overall dose with concentrations at demagnified areas (e.g. level B in FIG. 3), thereby avoiding errors due to dose transmitted at shielded areas (see peak at v=2 in FIG. 3 as an example, though such peaks can be yet larger in masks with repeating clear features). These areas contain residual dose partly because of Fresnel diffraction and partly because ( as in the next objective) absorbers are not, in practice, 100% efficient.
It is the tenth object of the invention to reduce CD errors in printed features by the use of Ultra High Resolution proximity lithography employing highly sensitive low overall dose with concentrations at demagnified areas, thereby avoiding errors due to variations in absorber thickness.
It is a eleventh object of the invention to apply some or all of the above objectives to projection lithography by exposing wafers on a plane off the true image plane.
It is the twelfth object of the invention to control the exposure dose when a mask contains features of various sizes by (a) multiple exposures with stepped gaps, (b) continuous exposures with varying gap or (c) multiple exposures with more than one mask.
It is the thirteenth objective of this invention to fabricate electronic, mechanical, magnetic or any other devices by the processes described in the above objectives.
The invention includes the exposure of lithographic images with ultra high resolution resulting from demagnification: printed feature dimensions on the wafer are smaller than those on the mask. Preferably, but not necessarily, the exposure is performed near the Critical Condition (defined below) with a development level similar to level B in FIG. 3.
The invention makes use of this ultra high resolution to reduce a range of defects in lithographic printing. The reduction is due to the averaging of intensities that arises from interference (or diffraction) in the construction of mask images during lithographic printing. As the rays travel through two dimensional mask features and interfere at one dimensional developed edges in the resist, there is an averaging effect which smoothes out defects of various kinds, the chief of which are identified below.
The invention also overcomes a difficulty in ultra high resolution lithography. Because, near the critical condition, the demagnification occurs by bias, the magnification is not uniform for features of different width. Nor, therefore, is the dose uniform. The invention overcomes this fact by various techniques adapted, as appropriate, with methods listed above in the twelfth object.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.